An organic dispersion type of an electric field luminous lamp (simply called "electroluminescence lamp" hereinafter) is widely utilized, especially, for a back-light device which is included in a liquid crystal display to illuminate letters and figures in various measuring apparatus, etc.
Such an electroluminescence lamp which is of a capacitive load is driven by an AC power supply having, for instance, a fixed voltage of 120 V and a frequency of 400 or 600 Hz.
However, a brightness of the electroluminescence lamp is decreased in proportional approximately to a driven time, because fluorescent material is not only deteriorated to lower a luminous efficiency, but an equivalent capacitive component of the electroluminescence lamp is also decreased to increase an impedance thereof due to the deterioration of the fluorescent material, thereby decreasing a current flowing through the electroluminescence lamp, so that an electric power consumption is decreased in a resistive component of the electroluminescence lamp.
For the purpose of improving the decrease of the brightness, a self-excited inverter of a blocking oscillation type, in which a DC voltage is inverted to an AC high voltage having a high frequency, is used to drive an electroluminescence lamp in place of the fixed AC power supply. The inverter has a property of a constant current, so that a voltage applied across the electroluminescence lamp is increased, when an impedance of the electroluminescence lamp is increased due to the deterioration of the fluorescent material.
For the same purpose, a separately excited inverter including a pair of transistors which are alternately turned on and off, a transformer for transforming a DC voltage switched by the transistors to a predetermined AC voltage, a choke coil connected between the transformer and an electroluminescence lamp is used for driving the electroluminescence lamp.
In the power supply circuit, the AC voltage is applied through the choke coil across the electroluminescence lamp by the transformer, so that the electroluminescence lamp is driven to emit light of a predetermined brightness, wherein a power-factor of the electroluminescence lamp which is ordinarily as low as approximately 0.25 due to the capacitive component is not only improved, but the decrease of the brightness is also compensated by a reactance component of the choke coil.
However, the former inverter, in which a frequency of the AC voltage applied across the electroluminescence lamp is determined by circuit constants and an impedance of the electroluminescence lamp, has a disadvantage in that the frequency becomes fluctuated, because a capacitance of the electroluminescence lamp changes in proportional approximately to a driven time thereof. As a result, the frequency has a possibility to synchronize with a driving frequency of a liquid crystal display, so that flicker is caused to occur in the display. In addition, it has further disadvantages in that beat is produced due to a magnetic distortion of a transformer, because the transformer is driven in its operating principle in the state of a magnetic saturation, and in that a loss is large to provide an efficiency which is generally as low as less than 60%, because an oscillation and the increase of a voltage are carried out by use of a single transformer.
Although the latter inverter does not have the disadvantage of the former inverter, in which the driving frequency of the electroluminescence lamp is fluctuated, it has additional disadvantages in that it is difficult to be small in size, because the transformer and the choke coil are indispensable, and in that a conversion efficiency is not high as expected, because a loss is large in regard to a conversion time.